Quercetin: benefits, dosage, contraindications
Other name(s)
Quercetol
Family or group:
Phytochemicals
Indications
Scoring methodology
EFSA approval.
High blood pressure ✪✪✪✪✪
A meta-analysis of predominantly small clinical trials shows that daily intake of 500 to 1000 mg of quercetin for 4 to 10 weeks reduces systolic blood pressure by 3.04 mmHg and diastolic blood pressure by 2.63 mmHg compared with placebo. rnrnThe benefit of quercetin on blood pressure appears modest.rnrn
Posologie
Oral
500 - 1095 mg
10 - weeks
Quercetin reduces blood pressure in hypertensive subjects.
Acute, quercetin-induced reductions in blood pressure in hypertensive individuals are not secondary to lower plasma angiotensin-converting enzyme activity or endothelin-1: nitric oxide.
Prostatitis ✪✪✪✪✪
Preliminary clinical research shows that taking 500 mg of quercetin twice daily for one month reduces pain and improves quality of life, but does not appear to affect urinary dysfunction in patients with chronic nonbacterial prostatitis. rnrnCombination with bromelain and papain may improve absorption and outcomes according to these studies. The anti-inflammatory, antioxidant, and immunomodulatory activities of quercetin may help explain these results. rnrn
Posologie
Oral
1000 mg
1 - month
Synergies
Quercetin's influence on exercise-induced changes in plasma cytokines and muscle and leukocyte cytokine mRNA.
Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial.
Allergies ✪✪✪✪✪
Quercetin is used in the treatment of acute and chronic allergic symptoms, such as hay fever and chronic rhinitis. Quercetin's anti-inflammatory activity and its ability to block allergic mediators justify its use for these indications. Quercetin inhibited histamine release by 46 to 96% in a dose-dependent manner in a study of 123 patients with mild to severe allergic rhinitis symptoms. rnrnFurther human trials are needed to more fully explore quercetin's potential to act as an antihistamine agent in various allergic conditions.rnrn
Posologie
Oral administration
2000 mg
Respiratory infections ✪✪✪✪✪
A clinical trial of 40 male cyclists receiving 1000 mg/day of quercetin or placebo for 2 weeks showed that the active treatment resulted in a significant reduction in the incidence of upper respiratory tract infections during the 2-week period after exercise. Another randomized double-blind clinical trial studied the effect of two different doses of quercetin (500 mg/day and 1000 mg/day) on the incidence and severity of respiratory infections in a large group of adults (n = 1002) aged 18 to 85 years. Overall, quercetin supplementation for 12 weeks did not have a significant effect on rates of respiratory infections or on symptoms compared with placebo. A reduction in the number of sick days as well as in the severity related to respiratory infections was observed in the group taking 1000 mg of quercetin per day.
Posologie
Oral administration
500 - 1000 mg
Quercetin Reduces Illness but Not Immune Perturbations After Intensive Exercise
Effectiveness of supplementation with quercetin-type flavonols for treatment of viral lower respiratory tract infections: Systematic review and meta-analysis of preclinical studies
Asthma ✪✪✪✪✪
Preliminary clinical research conducted on patients with mild to moderate asthma shows that taking quercetin at 250 mg or 500 mg per day for 30 days in combination with standard treatment reduces daytime and nighttime asthma symptoms, improves spirometry measurements, and decreases the use of rescue inhalers compared with baseline. Quercetin has also been used as an adjuvant in asthma management, often in combination with vitamin C because of its anti-allergic activity.
Posologie
Oral
250 - 500 mg
1 - month
Synergies
Cancer ✪✪✪✪✪
Quercetin is mainly found in fruits and vegetables, which have been shown to reduce the risk of certain cancers when eaten regularly. The anticancer effects of quercetin observed in the laboratory suggest a possible preventive role, and epidemiological studies have suggested potential benefits in the prevention of colon, lung (in smokers), pancreatic (in smokers), prostate, and breast cancers.
Posologie
Oral
40 mg
Quercetin Decreases the Expression of ErbB2 and ErbB3 Proteins in HT-29 Human Colon Cancer Cells
Dietary supplementation with the anti-tumour promoter quercetin: its effects on matrix metalloproteinase gene regulation
A Prospective Study of Dietary Flavonoid Intake and Incidence of Epithelial Ovarian Cancer
Quercetin, a Potent Inhibitor Against beta-catenin/Tcf Signaling in SW480 Colon Cancer Cells
Quercetin and Resveratrol Potently Reduce Estrogen Sulfotransferase Activity in Normal Human Mammary Epithelial Cells
Dietary Flavonoid Intake and Smoking-Related Cancer Risk: A Meta-Analysis
Overexpression of c-Jun Induced by Quercetin and Resverol Inhibits the Expression and Function of the Androgen Receptor in Human Prostate Cancer Cells
Flavonols and Pancreatic Cancer Risk: The Multiethnic Cohort Study
COVID-19 ✪✪✪✪✪
Two small open-label clinical trials conducted in Pakistan in symptomatic adults who tested positive for COVID-19 and whose oxygen saturation was above 93% evaluated quercetin as an adjunctive treatment to standard care including analgesics, antipyretics, and antibiotics. rnrnOne study shows that taking 200 mg three times daily for 7 days followed by 200 mg twice daily for 7 days appears to reduce the time to symptom resolution and to a negative COVID-19 test result, but does not prevent hospitalization, intensive care unit admission, or death, compared with standard care alone. rnrnAnother clinical trial in the same population shows that taking quercetin 200 mg twice daily for 30 days reduces the risk of hospitalization by 68%, the length of hospitalization (among those hospitalized) by about 5 days, and the need for supplemental oxygen by 93% compared with standard care alone. No patient receiving quercetin was admitted to intensive care or died, whereas 10.5% of patients in the standard care group were admitted to intensive care and nearly 4% died. rnrn
Posologie
Oral
400 - 600 mg
Possible Therapeutic Effects of Adjuvant Quercetin Supplementation Against Early-Stage COVID-19 Infection: A Prospective, Randomized, Controlled, and Open-Label Study
The therapeutic efficacy of quercetin in combination with antiviral drugs in hospitalized COVID-19 patients: A randomized controlled trial
Polycystic ovary syndrome ✪✪✪✪✪
Clinical research conducted in patients with PCOS shows that taking 1000 mg of quercetin per day for 12 weeks reduces testosterone and luteinizing hormone (LH) levels compared with baseline; both of these improvements are significant compared with placebo. Insulin resistance also improved by 18% compared with baseline, which was significant versus placebo.
Posologie
Orally
1000 mg
Effects of Quercetin on Adiponectin-Mediated Insulin Sensitivity in Polycystic Ovary Syndrome: A Randomized Placebo-Controlled Double-Blind Clinical Trial
The effects of quercetin supplementation on metabolic and hormonal parameters as well as plasma concentration and gene expression of resistin in overweight or obese women with polycystic ovary syndrome
Endometriosis ✪✪✪✪✪
In a controlled study carried out on 90 patients suffering from endometriosis, a combination of dietary supplements including 1002 mg of linoleic acid, 432 mg of alpha-linolenic acid, 200 mg of quercetin, 20 mg of nicotinamide, 400 mcg of folate, 20 mg of standardized turmeric, 19.5 mg of standardized parthenium, showed a significant reduction in symptoms at the end of treatment in the cohort of patients treated with the combination for three months, compared with the placebo group. Symptoms included: headaches, cystitis, muscle pain, irritable bowel, dysmenorrhea, chronic pelvic pain.rnrnIt was found that inflammation in endometriosis leads to an increase in cytokines by monocytes and macrophages, stimulating the production of PGE2 (prostaglandin E2) in endometriotic tissue. Thus, the reduction of PGE2 levels by certain dietary supplements could explain the clinical improvement observed in patients after treatment.rnrn
Posologie
Oral
200 mg
Synergies
Properties
Antioxidant
Quercetin's antioxidant effects likely play a role in most of its potential clinical effects. As a dietary flavonoid, quercetin has antioxidant effects like other flavonoids. rnrnPreliminary research shows that quercetin prevents the oxidation of low-density lipoproteins and may suppress the physiological reactions of heavy metal ions known to generate free radicals. Compared with curcumin, quercetin has a higher reducing potential with a total antioxidant capacity 3 times greater than that of curcumin. rnrnHowever, preliminary human research suggests that quercetin does not reduce biomarkers of oxidative stress, indicating that the antioxidant effects observed in vitro and in vivo may not translate into clinically meaningful benefits. Research also suggests that reduction of oxidative stress can only be expected in people with increased oxidative stress or inflammation, which may explain the negative outcome of many clinical studies in which antioxidants were given to healthy subjects. rnrnAdditionally, when acting as an antioxidant, quercetin can be converted into substances that release reactive oxygen species when they are converted back into quercetin. It can therefore have pro-oxidant activity, especially at high doses. It is suggested that in its protective process, quercetin depletes GSH levels and is transformed into potentially toxic products (quercetin metabolites reacting with thiol). rnrnIn the absence of GSH, potentially harmful oxidation products such as orthoquinone can be produced when quercetin exerts its antioxidant activity. Therefore, adequate levels of GSH should be maintained when quercetin is supplemented. rnrn
Chronic quercetin ingestion and exercise-induced oxidative damage and inflammation
A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity, including lack of genotoxic/carcinogenic properties
Quercetin reduces markers of oxidative stress and inflammation in sarcoidosis
Antiviral
Quercetin has demonstrated activity against retroviruses as well as against herpes simplex, polio, parainfluenza, and respiratory syncytial viruses (which cause respiratory infections). In vitro and in vivo evidence also suggests that quercetin can block replication of rhinovirus, which is responsible for the common cold. In cell cultures, quercetin reduces the ability of viruses to infect, likely by stimulating interferon production and by blocking certain proteins the virus needs to replicate. Quercetin's antioxidant effects may also protect the lungs from the harmful effects of free radicals during an influenza infection. Research on the effect of quercetin on SARS-CoV-2 (the virus responsible for COVID-19) indicates quercetin appears able to bind to viral enzymes, thereby blocking its entry into cells. In addition, its anti-inflammatory properties could help control and prevent the severe inflammatory response often seen in severe COVID-19 cases.
Usages associés
Antihistamine
Preliminary evidence suggests that quercetin may have an effect similar to that of cromolyn (antihistamine), by inhibiting antigen-stimulated histamine release from the mast cells of patients suffering from allergic rhinitis. rnrnOther research shows that quercetin may inhibit mast cell proliferation in vitro. Quercetin has been proposed as a useful treatment for allergic inflammatory diseases derived from mast cells, including contact dermatitis and photosensitivity, particularly in formulations with effective oral absorption. rnrn
Usages associés
Immunomodulator
In vitro research has shown that quercetin induces Th1-derived cytokines (promoting cellular immunity) and inhibits Th2-derived cytokines involved in allergies, which provides a theoretical basis for the use of quercetin as an anti-allergic substance. rnrnConversely, animal studies have demonstrated that quercetin is capable of inhibiting Th1 differentiation and interleukin (IL)-12 signaling and appears to exert an effect on Th1-mediated immune responses by suppressing the production of interferon-gamma and IL-2 cytokines. rnrnTherefore, it is possible that quercetin exerts an immunomodulatory effect on these cells.rnrn
Usages associés
Anti-inflammatory
In animal and in vitro studies, quercetin has shown an ability to reduce inflammation, notably by influencing neutrophil function and by inhibiting the production of certain inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and by modulating nitric oxide synthesis. However, human studies have not always confirmed these effects. A meta-analysis of 10 clinical trials found that quercetin did not significantly affect levels of C-reactive protein (CRP), interleukin-6 (IL-6) or TNF-α compared with placebo. Slightly positive results were observed in patients with diseases that have an inflammatory component, such as diabetes or metabolic syndrome.
Usages associés
Cardiovascular
The cardioprotective effects of quercetin may be related to its vasorelaxant, anti-inflammatory, and antioxidant properties and to the inhibition of proliferation and migration of vascular smooth muscle cells, as demonstrated by animal and in vitro models. rnrnIn humans, quercetin inhibits platelet aggregation and signaling as well as thrombus formation at a dose of 150 or 300 mg of quercetin glucoside. rnrnA small randomized controlled trial involving 12 healthy men and testing the effects of oral administration of 200 mg of quercetin, epicatechin, or epigallocatechin gallate on nitric oxide, endothelin-1, and oxidative stress showed that quercetin increased nitric oxide status and reduced endothelin-1 concentrations, thus suggesting the potential to improve endothelial function. Flavonoids such as quercetin also inhibit LDL oxidation, which is a key event in the genesis of atherosclerotic plaques. rnrnAlthough researchers believe that quercetin may protect against cardiovascular disease, short-term supplementation does not appear to affect cholesterol levels and its effects on blood pressure are mixed.rnrn
Usages associés
Anticancer
Quercetin may reduce cancer risk by inactivating malignant precursors or by inhibiting carcinogenesis. Epidemiological studies have shown that a high dietary intake of quercetin and related flavonols may reduce the risk of pancreatic cancer, particularly in male smokers, and may also reduce the risk of lung cancer. rnrnPreliminary studies suggest that quercetin may have inhibitory effects on various types of cancer, including breast cancer, leukemia, colon cancer, ovarian cancer, oral squamous cell carcinoma, endometrial cancer, stomach cancer, and lung cancer. rnrn
Usages associés
Hypoglycemic
Animal and laboratory research has shown that quercetin improves glucose metabolism. Diabetic rats receiving quercetin (15 mg/kg/day) for 4 weeks also experienced a decrease in blood glucose and an increase in plasma insulin, calcium, and magnesium. In human research, quercetin intake increased levels of adiponectin, a hormone involved in blood glucose control. Quercetin also decreases resistin, an adipokine involved in the induction of insulin resistance. rnrn
The effects of quercetin supplementation on metabolic and hormonal parameters as well as plasma concentration and gene expression of resistin in overweight or obese women with polycystic ovary syndrome
Safe dosage
Adults aged 18 years and older: 200 mg - 1000 mg
Quercetin has been used safely at doses up to 500 mg twice daily for periods of up to 12 weeks. rnrn
Interactions
Médicaments
Antidiabetic drugs: moderate interaction
Human research suggests that a combination of quercetin, myricetin, and chlorogenic acid reduces fasting glucose levels in patients with type 2 diabetes, including those taking antidiabetic medications. Although the effect of quercetin alone is not known, in theory concomitant use of quercetin and antidiabetic drugs could increase the risk of hypoglycemia. Dose adjustment may be necessary.
Antihypertensive drugs: moderate interaction
Quercetin may slightly lower blood pressure in people with mild hypertension. Theoretically, combining quercetin with other herbs and supplements that have hypotensive effects could increase the risk of hypotension.
Cyclosporine: moderate interaction
A small study in healthy volunteers shows that pretreatment with quercetin increases plasma levels and prolongs the half-life of a single dose of cyclosporine, possibly due to inhibition of P-glycoprotein or cyclosporine's metabolism by cytochrome P450 3A4 (CYP3A4).
Cytochrome P450 2C9: moderate interaction
A small clinical study in healthy volunteers shows that taking 500 mg of quercetin twice daily for 10 days before taking diclofenac increases diclofenac plasma levels by 75% and prolongs the half-life by 32.5%. Diclofenac is a substrate of cytochrome P450 2C9 (CYP2C9). Animal studies also show that pretreatment with quercetin increases plasma levels and prolongs the half-life of losartan, a CYP2C9 substrate. In addition, laboratory studies show that quercetin inhibits CYP2C9. rnrnTheoretically, using quercetin with drugs metabolized by CYP2C9 could lead to reduced drug clearance, increased serum drug levels, and increased effects.rnrnSome CYP2C9 substrates include celecoxib, diclofenac (Voltaren), fluvastatin, glipizide, ibuprofen (Advil), irbesartan, losartan, phenytoin, piroxicam, tamoxifen, tolbutamide, torsemide, and warfarin.
Diclofenac: moderate interaction
A small clinical study in healthy volunteers shows that taking 500 mg of quercetin twice daily for 10 days before taking diclofenac increases diclofenac plasma levels by 75% and prolongs the half-life by 32.5%. Co-administration of quercetin and diclofenac may increase diclofenac's effects and side effects.
Diltiazem: moderate interaction
In vivo pretreatment with quercetin resulted in an increase in the bioavailability of diltiazem, a calcium channel blocker, which may be the result of inhibition of P-glycoprotein and CYP3A4. Use with caution; dose adjustments may be necessary.
Midazolam: moderate interaction
A small clinical study shows that quercetin may increase the metabolism of midazolam.
P-glycoprotein substrate: moderate interaction
There is preliminary evidence that quercetin inhibits the gastrointestinal efflux pump P-glycoprotein. This inhibition could increase the bioavailability and serum levels of drugs transported by the pump, such as paclitaxel, diltiazem, losartan, cyclosporine, saquinavir, and digoxin. Use quercetin with caution when combined with P-glycoprotein substrates. Drugs transported by the pump include certain chemotherapeutic agents (etoposide, vinblastine, vincristine, vindesine), antifungals (ketoconazole, itraconazole), protease inhibitors (amprenavir, indinavir, nelfinavir), H2 antagonists (cimetidine, ranitidine), verapamil, corticosteroids, erythromycin, fexofenadine, loperamide, quinidine, and others.
Pravastatin: moderate interaction
In vitro evidence shows that quercetin can inhibit pravastatin uptake mediated by OATP 1B1. Additionally, preliminary evidence shows that taking quercetin at the same time as pravastatin increases the maximum concentration of pravastatin by 24%, prolongs pravastatin's half-life by 14%, and decreases pravastatin's apparent clearance by 18% in healthy men, suggesting that quercetin slightly inhibits pravastatin uptake into liver cells. Theoretically, concomitant use of quercetin and pravastatin could increase both the therapeutic effects and the side effects of pravastatin.
Quinolones: moderate interaction
Theoretically, quercetin could competitively inhibit quinolone antibiotics by binding to the DNA gyrase site on bacteria. Quinolones (fluoroquinolones) include ciprofloxacin, gemifloxacin, levofloxacin, moxifloxacin, and others.
Pioglitazone: moderate interaction
Quercetin may increase the bioavailability of pioglitazone. Due to the potential for toxicity, careful monitoring of liver and cardiac function is necessary.
Plantes ou autres actifs
Antihypertensive: mild interaction
Quercetin can lower blood pressure in people with mild hypertension. Theoretically, combining quercetin with other herbs and supplements that have blood-pressure-lowering effects could increase the risk of hypotension.
Quercetin: low interaction
Quercetin is an iron chelator and therefore should not be taken at the same time as iron supplements or iron-rich foods to avoid reduced absorption. Keep a 2-hour interval.
Precautions
Pregnancy: use with caution
Quercetin is widely available in the diet and is considered safe when consumed in dietary amounts (4 to 40 mg per day). rnrnThe safety of higher oral doses is unclear, as some research indicates that quercetin crosses the placenta and accumulates in the fetus, and relatively high intake of quercetin (200 to 1800 mg/day) may have harmful effects.rnrn
Breastfeeding: use with caution
No data available; use with caution at dietary doses.rnrn
Thyroid disorders: avoid
Quercetin may have some antithyroid properties, according to a study by Giuliani C et al. (2008). rnrnIt should be used with caution in people with thyroid disease.rnrn
